1. Field of the Invention
This invention relates to a vacuum-type circuit interrupter and more particularly to an improved construction of the contacts for increased current interrupting ability.
2. Description of the Prior Art
In the usual vacuum-type circuit interrupter, an envelope or housing is fabricated from a suitable insulating material such as glass, ceramic or the like. The housing forms a vacuum chamber in which a pair of separable contacts are disposed. In the closed position, direct electrical contact exists between the contacts of the interrupter and a continuous current path is established through the interrupter. In the open circuit position the contacts are spaced apart, forming an arcing gap therebetween. During operation, current interruption is initiated by separating the contacts. When the contacts separate, an arc is formed across the arcing gap. The arc which is formed vaporizes a portion of the contact material and these particles become ionized to help sustain the arc through which current flows until current interruption. Current interruption normally occurs at a current zero of the alternating current wave. After the current zero point has been reached, recovery voltage transients begin building up between the separated contacts. If the dielectric strength of the arcing gaps is sufficiently strong to withstand recovery voltage transients, breakdown will not occur, the arc will not reignite and circuit interruption will be complete. During circuit interruption, as the relatively movable contacts are separated an arc is formed therebetween and current will continue to flow through this arc until the arc is extinguished. The contact must support the arc from its initiation at contact separation until its extinction at approximately current zero. While the arc is being sustained the contacts are subject to very intense heating. The arc energy causes melting, erosion, and general deterioration of the contact surfaces. It is known in the prior art that contrate or cup-shaped contacts provide high current interruping ability. Slots formed in the cup-shaped contact can drive the arc around the electrode periphery and also drive the arc towards the center of the contact. When the arcs are forced radially inward, as disclosed in some prior art contact patents, they tend to grossly erode the inside base of the cup-shaped contact.
A prior art method of minimizing deterioration of the contact surface is with an axial magnetic field in phase with the arc current, reducing arc voltage and the power dissipated within the vacuum interrupter during the circuit interruption. It is known that the axial magnetic field in phase with the arc current can increase interruption capacity of vacuum interrupters. In the prior art, these fields have been created by coils inserted in the supporting stem of the contact or by external coil fields. In both cases, the coils conduct current even when the arcing contacts are closed and consequently represent a permanent energy loss due to the resulting eddy currents. It is desirable to have a contact which produces an axial magnetic field in place with the arc current only during arcing. It is also desirable to have a cup-shaped contact which has an inner base area which is not grossly eroded during high current interruption.